A San Luis Obispo city bus driver tried to drive a 14-foot-high double-decker bus under a 12-and-a-half-foot-high bridge for unknown reasons on Monday, destroying a portion of the bus’s upper level and halting traffic for about an hour.

The SLO Transit bus hit a railroad bridge sometime before 11 a.m. Monday morning as it attempted to go under the overpass on Highland Drive near Cal Poly. The bus was too tall for the posted 12-foot, 6-inch, clearance, and the collision sheared away the roof over the top level’s first two rows and smashed the front windows.

The bus was out-of-service and had no passengers at the time, SLO Transit manager Gamaliel Anguiano wrote in an email to The Tribune Monday afternoon. The driver “got off route,” and drove under the bridge, Anguiano wrote, though why is still under investigation.

“The bus driver was uninjured, but the bus sustained significant damage to the upper floor and has been transported back to the SLO Transit yard for review and eventual repairs,” he wrote.

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The incident stopped traffic on Highland Drive into and out of Cal Poly while authorities attempted to move the bus from the roadway. The road was reopened by noon, and normal traffic resumed.

This is not the first time a too-tall vehicle has struck the bridge. In September 2009, a truck hauling a 45-foot-long trailer with a shipment for the school’s dairy unit got stuck under the overpass for several hours. Additionally, in August 2013, a semi-truck tried to drive under the bridge, crumpling the posted clearance sign.

The bridge is a rail crossing for Union Pacific Railroad. According to Anguiano, railroad representatives were on the scene soon after the crash Monday to assess damage to the bridge, though initial review indicated that most of the damage was to the vehicle, not the railway. No train traffic on the rail line through San Luis Obispo was affected.

Union Pacific representatives did not respond for comment Monday afternoon.

SLO Transit unveiled the $844,000 double-decker bus in September 2010, as a way to reduce carbon emissions and attract new bus riders.

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